1. Field of the Invention
The present invention relates to a member made of an Ni base alloy having a high resistance to stress corrosion cracking, suitable for use under an atmosphere of a temperature below creep temperature, particularly in contact with water of high temperature in various plants treating high temperature water such as boiling water reactors or pressurized water reactors. More particularly, the invention relates to various parts made of the Ni base alloy such as retainer beam of jet pump for nuclear reactors, springs and bolts used in the nuclear reactors and so forth. The invention is concerned also with a method of producing such parts.
2. Description of the Prior Art
An alloy generally called inconel X750 (referred to as X750 alloy, hereinafter), i.e. Aerospace Material Specification (AMS) 5667H, which is an Ni base alloy of the precipitation strengthening type having a high modulus of elasticity and a large high-temperature strength, finds a spreading use as the material of various parts in nuclear reactors, such as retainer beam of jet pump, springs, bolts and so forth. This X750 alloy has a Cr content of around 15% and is usually regarded as being a corrosion resistant material. According to the result of studies made by the present inventors, however, it has been proved that the X750 alloy often occurs stress corrosion cracking when used in contact with water of a high temperature such as the water circulated through nuclear reactors, depending on the nature or quality of the water. More specifically, the X750 alloy tends to exhibit an intergranular stress corrosion cracking when it is subjected to a pure water of a high temperature of about 290.degree. C. under a condition subjected to tensile stress, particularly when there is a crevice in the surface onto which the tensile stress acts.
The specifications of USSN 1967-653665 and USSN 1965-459110 disclose Ni-base alloys having a high resistance to stress corrosion cracking suitable for use in contact with highly pure water of high pressure and temperature, as in the case of pressure vessel type heat exchangers, steam generator and so forth. More specifically, the specification of USSN 1967-653665 discloses an alloy consisting essentially of 14 to 35% of Cr, 0 to 25% of Fe, less than 0.5% of one or both of Ti and Al, 0 to 15% of C, 0 to 1% of Si, 0 to 7.7% of Mo, 0 to 1.2% of Ta and the balance Ni, wherein the Cr content is less than 20% when the alloy has a substantial Mo or Ta content. On the other hand, the specification of USSN 1965-459110 discloses an improvement in the Ni base alloy mentioned above, consisting essentially of 26 to 32% of Cr, less than 0.1% of C, less than 5% of Ti, less than 5% of Al, less than 2% of Mn, less than 2.5% of Si, 52 to 67% of Ni and the balance Fe, and an alloy containing, in addition to the constituents mentioned above, at least one of less than 10% of Mo, less than 6% of Nb, less than 10% of V and less than 10% of W.
The alloys disclosed in these literatures, however, proved to have insufficient strength against the crevice corrosion cracking in the aforementioned parts forming a crevice therebetween.